My dell 519 desktop motherboard died. Can I use an after market one or do I need to use an original?
You'll need to review the inside of your case to determine if the motherboard is a standard M-ATX or ATX case and Dell didn't use unique front panel connectors that may not work on another motherboard.
If nothing looks "unique" / Dell specific, you should be able to upgrade an aftermarket motherboard. Granted the I/O Panel may not work in the case, so it might have to be left off, which isn't a major deal...